1. Field of the Invention
The invention relates to a fuel cell system and a control method for a fuel cell system and, more particularly to a technique for precisely setting the flow rate of air supplied to a fuel cell using a low-cost air flow meter.
2. Description of the Related Art
A fuel cell generally generates electricity by, for example, causing electrochemical reaction between air used as oxidant gas and hydrogen used as fuel gas. However, when the flow rate of air, as oxidant gas, supplied to the cathode of the fuel cell is insufficient, the power generation efficiency decreases. In addition, an electrolyte membrane, which is an important component of the fuel cell, normally has a property that the power generation efficiency increases in a humid state. Therefore, when the flow rate of air supplied is excessively high, the flow of air decreases the humidity of the electrolyte membrane, so the power generation efficiency decreases. For this reason, it is important to measure the flow rate of air supplied to the cathode of the fuel cell with an air flow meter (AFM) and then to control the flow rate so as to constantly become an appropriate state.
Then, development of a vehicle equipped with the fuel cell (fuel cell vehicle) has been pursued. In order to spread the use of the fuel cell vehicle, it is important to reduce the cost of components of the fuel cell system, and the above described air flow meter is also an important element for cost reduction. Japanese
Patent Application Publication No. 2004-253208 (JP-A-2004-253208) describes a technique for calculating the air flow rate using a pressure sensor in a fuel cell system. Japanese Patent Application Publication No. 2008-091337 (JP-A-2008-091337) describes a fuel cell system that uses an air flow meter to execute air flow rate feedback control as a function of an actual air flow rate and a required air flow rate.
However, a low-cost air flow meter definitely has a decreased measurement accuracy as compared with an expensive air flow meter. This becomes a bottleneck of high-precision supply flow rate control. In addition, an increase in measurement error due to aged degradation of an air flow meter also becomes a bottleneck of high-precision supply flow rate control.